The subject matter disclosed herein relates generally to imaging systems, and more particularly, to pinhole collimators for nuclear medicine imaging systems and determining a system matrix for the pinhole collimator imaging systems.
Nuclear medicine imaging systems, for example, single photon emission computed tomography (SPECT) imaging systems, use one or more image detectors, sometimes many image detectors, such as gamma cameras to acquire image data (e.g., gamma ray or photon image data). Collimators are used in combination with the image detectors to select the direction from which incident gamma rays are accepted and reduce the effects, for example, of radiation from other parts of the body that can degrade image quality (e.g., cause image artifacts). Thus, collimators can improve spatial resolution.
Nuclear imaging systems with gamma cameras and pinhole collimators are increasingly being used for small animal and organ specific imaging in humans. A point spread function (PSF) of the gamma cameras is used to describe the photon count density distribution at the detector surface when a point source is imaged. Accurate modeling of the PSF is important for performing accurate image reconstruction, for example, of SPECT images. Accordingly, accurate modeling is important for resolution recovery, as well as for improving the quantitative accuracy of the reconstructed image. However, accurately determining the PSF of pinhole collimators is challenging as the PSF is a function of source location (shift-variant). One factor that contributes to the shift-variant nature of the PSF is the penetration of photons through the pinhole aperture.
Conventional reconstruction algorithms are either ray-driven or voxel driven. In these reconstruction algorithms, the PSF of the pinholes are usually modeled using a simpler shift-invariant PSF. The simplifications can cause distortions in the reconstructed images, as well as affect the quantification in the images. Different methods are also known to calculate a system matrix for a nuclear medicine imaging system. The system matrix generally defines the physics of the system. The known methods perform physical measurements to determine the system matrix. The measurements are acquired by moving a point source to different locations in the image space and saving multiple acquired projections. However, in order to obtain sufficient counts in the projection data, the total acquisition time to calculate the system matrix can be from four hours up to eighteen hours. In order to speed up the process, the system matrix is sometimes measured for intermediate points (e.g., 400 intermediate points) and the system matrix for the intermediate grid is determined using interpolation. This process is not capable of exactly determining the PSF for any point in the image space and can introduce errors.
Other known Monte-Carlo based methods are used wherein the pinhole is assumed to be formed from discrete steps. The photon flux through the pinhole aperture, as well as the collimator material, is then measured and stored as a system matrix. However, this method is computationally demanding and time consuming, resulting in a slow process that can also have discretization errors. The accuracy of the system matrix depends greatly on the model used to define the pinhole aperture.